SUMMARY This request for an administrative supplement to increase funding is submitted in response to PA-18-591 (Administrative Supplements to Existing NIH Grants). We request funding to complete work within the original scope of the parent award and to finish a manuscript currently under development, which addresses how the expression of miR-21 and miR-181b is differentially regulated in early and late sepsis. My current R01 GM103887 is the parent award for this supplemental request. The research in my parent award focused on the roles of myeloid-derived suppressor cells (MDSCs), which suppress both innate and adaptive immunity, in the pathogenesis of sepsis. Sepsis, a highly lethal and common disease, is initiated by an early systemic hyperinflammatory reaction, which rapidly shifts to a late/chronic immunosuppressive state. We have shown that MDSCs expand dramatically and suppress both innate and adaptive immunity in sepsis. Eliminating this deletrious cell population rescued mice from chronic sepsis immunosuppression and death. Our work demonstrated that miR-21 and miR-181b coupled with the transcription factor NFI-A to promote MDSC expansion. A major focus of this research was investigating the mechanisms that induced these miRNAs in sepsis. This work was described in Specific Aim 2 of the parent award. We have successfully uncovered the mechanism of miRNA induction in sepsis. We reported that the transcription factors STAT3 and C/EBP? synergize to activate miR-21 and miR-181b promoters and expression in sepsis. We confirmed these findings using a conditional, myeloid-specific knockout mouse model lacking C/EBP?, as they lack miR-21 and miR-181b expression and do not generate MDSCs. Now, using supplementary funding, we seek to complete our new preliminary data and extended concept, which suggest that the miRNA promoters are differentially controlled during early and late/chronic phases of sepsis. We recently used a knockout mouse model lacking the inflammatory S100A9 protein to show that these mice survive sepsis. Importantly, they lacked the miRNA expression only in late sepsis and did not generate MDSCs. Intriguingly, these mice still express miR-21 and miR-181b, and expand Gr1+CD11b+ cells (the precursors of MDSCs) in early sepsis despite normal expression of STAT3 and C/EBP?. These unexpected findings suggest that an additional mechanism requiring S100A9 regulates miRNA expression during the late/chronic phase of sepsis. We will define how S100A9 supports miR-21 and miR-181b expression. This supplement will allow us to complete unfinished research and support our planned manuscript and publication to report how miR-21 and miR-181b are differentially regulated after sepsis transitions from an early proinflammatory response to a more chronic state of sustained immune suppression. Understanding the mechanisms of MDSC expansion and sepsis-induced immune suppression may inform new ways to treat chronic sepsis with immune suppression.